The invention relates to an angle guide for orienting a bone screw in a bone plate. More particularly, the invention relates to an instrument for insuring the position of the head of a bone screw located within a bone plate hole.
It is often necessary when utilizing bone plates either for fracture fixation or such as tibial base plates to couple the bone plates to bone via a screw extending through a hole in the plate. When the screw is angled, surgeons have had to eye the correct angle specified by the device manufacturer indicated by the surgical situation. If the bone screw is put in at too great an angle the bone screw head would sit proud of the bone plate which in some situations is undesirable. The surgeon would then either continue to torque the bone screw until the bone screw's cutting flutes soften the bone in the area and allowed it to sit properly or the bone screw would be backed out and the surgeon would attempt to reinsert the bone screw at a shallower angle.
The surgeon often had difficulty eyeing the correct angle especially where there was a plurality of bone screws to be inserted, since the correct angle is relatively small i.e. zero to ten degrees. Normally, the surgeon drills a pilot hole in the bone prior to inserting the bone screw and it has been difficult to drill the pilot hole at the correct angle so that the bone screw would seat properly. For example, when using a tibial base plate, it is typical to utilize a polyethylene bearing surface snapped onto the base plate which has been implanted on a prepared tibia. Consequently, if the pilot hole and subsequent bone screw are inserted at an incorrect angle the head of the bone screw could impinge on the distal surface of the polyethylene bearing implant which is undesirable. Such an impingement could lead to metal or polyethylene debris which has been known to cause osteolysis.
If the surgeon saw that the bone screw did not seat properly or was not at the specified depth, he could continue to torque the bone screw. However, since most bone screws include cutting flutes at the leading ends thereof continued rotation of the cutting flutes causes the bone to degrade and allows more play in the screws positioning. While this may allow the bone screw to seat properly, it reduces the compressive forces holding the plate to the bone by decreasing the holding ability of the screw.
Alternately, if the surgeon noticed the bone screw did not seat properly or is not at the correct depth, he might reverse the screw and drill a new pilot hole at a slightly different angle. This method may not always be successfully since the drill and screw will have a tendency to follow the original drilled hole. Even if the surgeon is capable of resetting the new desired angle, there is still damage to the bone in the originally drilled area which may cause fracture propagation. Consequently, there has been a long felt need to provide an instrument which can indicate the angle which is not to be exceeded for proper positioning of the bone screw in the bone plate. This instrument will verify that the final implant positioning is accurate and allows the head of the bone screw to be properly located.